Bonding of lignocellulosic materials, such as wood, is widely used commercially, such as in the manufacture of plywood and board from particulate wood. In present commercial bonding procedures, adhesives, such as urea- or phenol formaldehyde are employed, which are spread or otherwise applied to the surface of the material, and penetrate the wood structure whereby bonding is effected by the adhesive.
Procedures have been proposed to effect such bonding by chemical reactions between reagents and the wood itself, but have not met commercial acceptance. Thus, U.S. patents in the names of Willey, et al U.S. Pat. No. 2,495,043 and Wilson, U.S. Pat. No. 2,639,994, suggest the treatment of wood with acid, followed by pressing. Neither process achieves satisfactory results, i.e. bonding of adequate strength, possibly because, at any reasonable pressure, i.e. a pressure at which the cellular structure of the wood is not collapsed, only a fraction of the surfaces to be bonded come into contact because of the uneven and cellular character of the wood surface.
Acids have an hydrolytic effect upon cellulose of wood and thus cause permanent loss in strength. For example, treating of wood in autoclave with 0.5% hydrochloric acid at 9 atmospheres (132 psi) pressure which correspond to 348.degree. F. temperature for 15 minutes yields 22% of simple sugars by weight of the wood. Hydrolysis of wood carbohydrates to sugars results in significant loss of strength because, besides partial transformation of carbohydrates to sugars, unhydrolyzed carbohydrates are depolymerized to a high degree which means splitting to shorter molecular chain fragments. It is not possible to confine this hydrolytic degradation to a thin surface layer with the intention of producing a "bonding layer" without affecting adjacent layers. Those adjacent layers are always at least partially degraded with significantly decreased wood strength.
Likewise, Salisbury U.S. Pat. No. 2,204,384, does not produce bonding which is resistant to cold or hot water. Moreover, bonding adhesive is applied in cold conditions. Starch and sucrose are stabilized by non-volatile acids, mixed at moderate temperatures, then cooled to produce a water-soluble adhesive in powder form which is thereafter diluted in water before use.